This invention relates generally to carrier signal recovery loops located at a receiver for the reception and demodulation of quadraphase shift keyed (QPSK) signals, and more particularly to such a carrier recovery loop for the demodulation of QPSK signals which are received intermittently in a burst mode. Such carrier recovery is one of several functions which together comprise the demodulation process by which signals received on a radio frequency carrier are detected and converted to baseband frequencies.
In some prior art methods of recovering the carrier at the receiver for demodulating a QPSK signal the frequency f.sub.c of the carrier signal e.sub.c of the QPSK signal is multiplied by four to remove the four phase modulation, leaving a line frequency sine wave component of 4f.sub.c. This sine wave signal is then employed as one input to the phase detector of a phase locked loop (PLL) circuit to become phase locked with the phase of a multiplied-by-four frequency (4f.sub.vco) of a locally generated carrier signal e.sub.vco of a frequency f.sub.vco and generated in a voltage controlled oscillator (VCO).
The recovered carrier e.sub.vco of frequency f.sub.vco will then have one of four phase relationships, 0.degree., 90.degree., 180.degree., or 270.degree. with the phase of the received carrier e.sub.c of frequency f.sub.c. In a subsequent step in the demodulation process (not a part of the present invention) this phase ambiguity is resolved to obtain bit synchronization and detection.
In applications employing bursts of QPSK transmission, the received carrier will generally deviate from the phase locked condition between bursts so that when the next burst occurs the locally generated carrier signal is no longer phase locked with the carrier of the received signal and accordingly must be phase corrected before the demodulation of the next transmission burst can be accomplished.
When the system is operated in the burst mode of operation, it is assumed herein that each transmission burst will begin with a synchronizing preamble as is normally done in time division multiplex access (TDMA) transmissions. For the purpose of this invention it is further assumed that a portion of the burst preamble will consist of a sequence modulated by alternating symbol phases of zero and .pi. radians. Such a two valve sequence produces a line frequency component at 2f.sub.c after frequency doubling (or 4f.sub.c after frequency quadrupling) and permits the phase locked loop to initiate synchronization in the present invention.
As an example of the problem that can be encountered between transmission bursts assume that the phase of the VCO signal has wandered off the phase locked value by some phase between 0.degree. and 360.degree. of a cycle of the desired frequency f.sub.c. As will become clearer later herein, the worst possible phase shifts that can occur between transmission bursts are even multiples of 45.degree. of a cycle of f.sub.c which would establish a phase relation between the quadrupled (x4) frequency 4f.sub.vco of the locally generated signal e.sub.vco and the quadrupled (x4) frequency 4f.sub.c of the received carrier signal e.sub.c at phase angles of either 45.degree., 135.degree., 225.degree. or 315.degree., relative to f.sub.c. All of these phase relations represent an unstable null point at the output of the phase detector which compares the phases of the quadrupled frequencies 4f.sub.c and 4f.sub.vco of e.sub.c and e.sub.vco. Thus, the phase of the quadrupled frequency 4f.sub.vco of the local VCO signal e.sub.vco must be shifted by a full 180.degree. of a cycle in order to become phase locked with the quadrupled frequency 4f.sub.c of received signal e.sub.c at one of the four possible stable null points which occur at any one of the phase relations 0.degree., 90.degree., 180.degree. or 270.degree. between e.sub.vco and e.sub.c.
The amount of time required for the quadrupled frequencies of e.sub.c and e.sub.vco to become phase locked at one of the above-mentioned four stable null points presents the specific problem which is met and solved by the present invention, particularly when the loop is initially near an unstable null. More specifically, it is a substantial reduction in the time required to acquire phase lock starting from the vicinity of one of the four unstable null points that the present invention achieves.
It should be specifically noted, as mentioned above, that the resolution of the ambiguity presented by the four stable null points, i.e., the selection of the proper one of the four stable null points, is not a part of the present invention. This problem is solved using such well-known techniques as differential coding of the transmitted data or the detection and correlation of coded words that are a part of the burst preamble mentioned above and which also serve to provide word synchronization.
The above-mentioned techniques are discussed in detail in the following three publications, all of which are incorporated in full herein by reference.
1. "Digital Communications--Satellite/Earth Station Engineering", pp 306-7, 385-6, Prentice-Hall 1983.
2. "Preamble Requirements for Burst-Type QPSK Satellite Communications under Low Es/No Conditions", by S. A. Rhodes, Proceedings of 1977 National Telecommunications Conference, pp 05:3-1 to 05:3-7.
3. "Phase-Ambiguity Resolution in a Four-Phase PSK Communication System", by E. R. Cacciamani and C. J. Wolejsza Jr., IEEE Trans. on Comm. Technology, Vol. COM-19, No. 6, Dec. 1971, pp 1200-1210.
After correction of the output of the local VCO to the proper one of the four possible stable null points of the x4 frequencies, the output e.sub.vco of the local VCO is compared with e.sub.c for the demodulation function.
An unstable null point derives its label of instability from the fact that any change in the phase difference between the two signals being phase compared will produce a change in voltage at the output of the phase detector of a polarity which will increase such change in phase difference between the two signals, thereby increasing the phase difference until the phases of the two signals coincide, i.e., until a stable null point is reached, as at 0.degree.. At a stable null point, any change in phase between the two signals from the stable null point will result in an output voltage from the phase detector which will cause the phase of the output of the VCO to change in a direction as to close the phase difference between the two signals, i.e., to return to the stable null.